It will be a late, languid event. For people living east of the Mississippi, the eclipse begins well after midnight. The lunar eclipse will last about three and a half hours, with the moon falling in the depths of Earth's shadow for about an hour and twelve minutes at the height of the event.

As Earth slips between the sun and moon, changing the tint of the lunar surface from white to orange to russet and back, you're seeing the effect Earth's atmosphere is having on the color of sunlight passing through it. But the atmosphere is doing something else. It's in effect tagging the sun's rays with the chemical fingerprints of gases in the atmosphere.

Over the past two years, two teams of astronomers have been using this effect to figure out what Earth might look like as a distant, extrasolar planet orbiting another star. By analyzing the light reflected off the moon during a lunar eclipse – light that has passed through Earth's atmosphere – they have detected gases in the atmosphere that indicate the presence of organic life on the planet.

If the teams' baby steps are any indication, the techniques they are developing may be able to detect evidence of organic life imprinted in an extrasolar planet's atmosphere – at least for rocky, Earth-mass planets orbiting stars relatively close to the sun – using large Earthbound telescopes.

"It's an exciting experiment – one of the few I've seen that I wish I'd thought of myself," says Sara Seager, a physicist at the Massachusetts Institute of Technology who studies exoplanets and their atmospheres and who was not involved in either project.

"The Earth is our best laboratory; it's the only planet we know of with life," she says. "So we really want to understand what Earth would look like as an exoplanet far away."

These are the types of extrasolar planets NASA's Kepler spacecraft and the French Space Agency's CoRoT spacecraft currently are hunting.

Kepler in particular is searching more than 150,000 stars for Earth-mass planets in their stars' so-called habitable zones. These are regions of space close enough to a star that liquid water would be stable on the surface of a planet orbiting at that distance.

Chemical signature

As a transiting planet passes in front of its star, starlight passes through the planet's atmosphere, picking up spectral signatures of atoms and molecules there and carrying those signatures with it as the starlight continues to travel.

For a team led by astronomers Enric Palle, with the Instituto de Astrofisica de Canarias, at Tenerife on the Canary Islands, the question was: What would Earth's atmosphere look like to distant astronomers watching the planet transit the sun?

Since the team couldn't travel far enough away to observe the Earth as a transiting planet, the researchers enlisted a full lunar eclipse in August 2008 as a stand-in.

From the moon's perspective during an lunar eclipse, Earth is a transiting planet. It blocks direct sunlight that otherwise would shine on the moon.

But the moon still receives and reflects sunlight that passes through Earth's atmosphere from the daylit half of planet. An observer on the moon would see a dark disk ringed by a thin, brilliant, sunset-like band of orange and red.

Dr. Palle and his colleagues posited that this light, reflected back to Earth from the moon's surface, would carry the spectral signatures of molecules in Earth's atmosphere.

The last time astronomers tried to pick out gases in Earth's atmosphere from earthshine during an eclipse nearly a century ago, the technology wasn't up to the task.

This time around, Palle's team used spectrometers bolted to the backs of two telescopes in different parts of the world – one for visible-light measurements and one for near-infrared measurements. The team was able to spot the signatures of carbon-dioxide, water, methane, ozone, and molecular oxygen in the dusk-like sunlight the moon reflected.

Smoking gun

That's a combination of gases that exobiologists say would represent a smoking gun in the hunt for extrasolar planets likely to harbor life.

The results, published in the journal Nature in Nov. 2009, were encouraging. But while the results gave the team a sense for what Earth's atmosphere contained, they didn't have much to say about details regarding the abundance of those gases.

That's where a team led by Alfred Vidal-Madjar with the Astrophysics Institute of Paris picks up the story. The team observed the same lunar eclipse, but with a spectrograph that not only recorded the chemical fingerprints in more detail than did the first team's instruments. It also could pinpoint the spots on the moon from which it was taking its measurements.

This ability to measure the light simultaneously from areas of different brightness as Earth's shadow inched its way across the lunar surface, gave the team some additional information that allowed the scientists to estimate the thickness of the atmosphere as well.

"We are confident that quantitative information about extrasolar atmospheres will be within reach" as a new generation of very large, ground-based telescopes begins operation equipped with spectrometers similar in design to the one they used begin operation, the researchers wrote when they reported the results in November in the journal Astronomy and Astrophysics.